Anionic surfactants have been utilized as the major actives in many skin cleansers. Notwithstanding their many advantages (e.g., having good lathering properties), they tend to irritate skin. For example, irritated and cracked skin often result from the use of fatty acid soap, especially in colder climates. One method of reducing the harshness of anionic surfactants in general (including fatty acid soap) is to utilize other surfactants, such as amphoteric surfactants, as coactives to partially replace anionic surfactants in skin cleansing products. While not wishing to be bound by theory, it is believed that amphoterics reduce the skin irritation by forming colloid aggregates (micelles, vesicles and liquid crystals) with the skin-irritating anionics in aqueous personal washing liquor, which hinders the penetration and binding of the anionic surfactants to the skin proteins.
The use of amphoteric surfactants in solid, skin cleansing bars, however, can introduce problems in bar processing and user properties. For example, introducing 10% to 15% wt. of cocoamidopropyl betaine (a commonly used amphoteric surfactant) to an extruded synthetic surfactant bar results in a formulation which is sticky and thereby severely slows down the extrusion throughput. Including the same level of cocoamidopropyl betaine in a fatty acid soap based bar increases time cycles in mixing and drying. Most amphoteric surfactants are sticky (gelish) and sensitive to work (e.g., thinning/gelling in response to shear). These properties slow down or even stop the extrusion/plodding, cause stickiness to the stamping die, and tend to give undesired mushiness and softness to bars. Further, many of these amphoteric surfactants are difficult to dry into low moisture solids (e.g., powders or pellets). Therefore they are commercially supplied in the form of diluted aqueous solutions, which brings in extra amount of water into the mixer and lengthens the mixing-drying time.
There is thus a need in the art for amphoteric surfactants that are non-sticky, have a low moisture solid state, can be used to reduce the mixing/drying cycle, and can be continuously processed by the extrusion/plodding technology at high throughput. High levels of said amphoteric surfactant should be able to be incorporated into extruded bars (containing either synthetic surfactants or fatty acid soap or mixtures thereof) without causing processing difficulties and negatively affecting bar user properties such as lather and bar hardness. Preferably, bar user properties (e.g., lather) should be enhanced by the inclusion of said amphoteric surfactant.
It is also desirable to identify amphoteric surfactants which may be more efficient than other amphoterics in reducing skin irritation caused by the anionic surfactants or fatty acid soap in bars.
It should be noted that bars containing synthetic surfactants have a different formulation space when compared with fatty acid soap bars. While bars containing synthetic surfactants require additional structurants such as fatty acids and waxes, fatty acid soap bars do not. The processing procedures for synthetic surfactant bars and fatty acid soap bars also have many differences, as described in many patents covering the field.
Therefore, identifying an amphoteric surfactant that simultaneously meets the needs listed above for both synthetic and fatty acid soap bars is extremely technically challenging. Unexpectedly, however, applicants have found that amphoterics defined by certain physical parameters meet these needs.
The use of solid amphoteric surfactants (e.g., disodium N-lauryl iminodipropionate) in bar and liquid compositions is not itself new. This amphoteric surfactant, for example, has been incorporated in acidic, low pH bars containing synthetic anionic surfactants. The disodium N-lauryl iminodipropionate was applied to elastic, rubbery bars prepared using a cast melt process. It has also generally been used as a mild detergent in liquid cleansers (e.g., shampoos and liquid body washes).
U.S. Pat. No. 3,442,812 to J. Barnhurst et al. (assigned to Colgate-Palmolive Co.) teaches a non-soap, synthetic detergent bar with an acidic lather having skin conditioning effects. The bar lather has to be acidic with pH less than 6 (i.e., pH at 5 or below as described in column 2, line 42-68, and claim 1, 12, 13 of said patent). Disodium N-lauryl iminodipropionate is cited as one of the amphoteric surfactants used. The patent did not recognize the criticality of using a solid amphoteric surfactant in bar formulations to improve the processing. Further, it does not recognize the superiority of disodium N-lauryl iminodipropionate (or other amphoterics having physical parameters defined by the subject invention) when compared with other amphoterics in reducing the skin irritation caused by anionics. The requirement for low pH also prevents the use of fatty acid soap (pH&gt;7) as the bar ingredients in this application. By contrast, the amphoterics defined by the subject invention (e.g., disodium N-lauryl iminodipropionate) can be used as solid coactives in both fatty acid soap and synthetic surfactant based extrusion bars. The bars of the invention must have a neutral or basic pH (i.e., between 6 and 12, preferably between 6 and 10, and most preferably between 6.5 and 9). The subject application teaches the use of these solid amphoteric surfactant to (1) achieve processing improvements; (2) achieve superior skin mildness as compared with other amphoteric surfactants. These attributes are neither taught nor suggested in the referred patent.
U.S. Pat. No. 4,080,310 to L. Ng et al. (assigned to Beecham Group, Ltd.) teaches an amphoteric conditioning shampoo, which contains 5 to 50% w/w of amphoteric detergent as sole detergent and 0.5 to 3.0% w/w of cationic or quaternary resin. The pH is from 3 to 9, preferably 4 to 7. The amphoteric detergent may be, for example, an N-alkyl-.beta.-aminopropionate or N-alkyl-.beta.-iminodipropionate. Suitable resins are cationic polyamide polymers or a cationic starch or cellulose derivatives. Said patent does not teach the use of solid amphoterics as defined (e.g., disodium N-lauryl iminodipropionate) in skin cleansing bars for the advantages of processing and simultaneously reducing the anionic irritation. In contrast, in the subject application, disodium N-lauryl iminodipropionate is incorporated in synthetic surfactant and/or fatty acid soap based extrusion bars to (1) facilitate the bar processing; (2) enhance the mildness of the bar formulation; and (3) enhance the creaminess of the bar lather performance.
U.S. Pat. No. 4,207,198 to D. Kenkare (assigned to Colgate-Palmolive Company) teaches an elastic detergent bar of improved form-retaining ability during elevated temperature storage and of improved foaming power. The bar comprises an organic detergent, which is an ammonium or lower alkanol-ammonium anionic organic detergent salt, or a mixture of such anionic detergent with amphoteric synthetic organic detergent, gelatin and a lower di- or polyhydric alcohol. The amphoteric detergents claimed include N-alkyl-.beta.-iminodipropionate. The bars are prepared by a cast-melt method and display an extensive degree of elasticity. The rubbery bar is described in the claim 1 as "2 cm thickness thereof can be pressed between a thumb and forefinger to a 1 cm thickness and upon release of such pressure will return within five seconds to within 1 mm of the 2 cm thickness". In contrast, the amphoterics of the subject invention (e.g., disodium N-lauryl iminodipropionate) are used in bars prepared by the extrusion method, which requires extrudate having rigidity and solid nature. Most importantly, incorporating the solid amphoteric surfactant in the extrusion bars help reduce the bar softness and elasticity. Therefore the referred patent teaches away from the art of the subject application.
U.S. Pat. No. 4,328,131 to J. Carson et al. (assigned to Colgate-Palmolive Company) teaches an elastic, rubber-like detergent bar (described as "2 cm thickness thereof can be pressed between a thumb and forefinger to a 1 cm thickness and upon release of such pressure will return within five seconds to within 1 mm of the 2 cm thickness" in the claim 1) of improved elevated temperature stability, so that it better maintains its shape on storage at temperatures somewhat higher than normal, includes an amphoteric synthetic organic detergent in mixture with an anionic synthetic organic detergent, gelatin, water and insoluble gas in very small bubble form distributed throughout the bar. The amphoteric surfactants used include disodium N-alkyl-.beta.-iminodipropionate. Bars are prepared by the cast-melt method. In contrast, disodium N-lauryl iminodipropionate is used by the subject application in bars prepared by the extrusion method, which requires extrudate having rigidity and solid nature. Most importantly, incorporating the solid amphoteric surfactant in the extrusion bars is for reducing the bar softness and elasticity. Therefore the referred patent teaches away from the art of the subject application.
U.S. Pat. No. 3,962,418 filed to R. Birkofer teaches a mild, thickened liquid shampoo composition with conditioning properties comprising 4-8% anionic surfactants, zwitterionic and amphoteric surfactants, polyethoxylated nonionic surfactants and a cationic cellulose ether thickening and conditioning agent. The amphoteric surfactants used include disodium N-alkyl-.beta.-iminodipropionate. However, said patent does not teach the use of solid disodium N-lauryl iminodipropionate in solid skin cleansing bars for the advantages of bar processing and simultaneously reducing the skin irritation. In contrast, in the subject application, disodium N-lauryl iminodipropionate is incorporated in synthetic surfactant and/or fatty acid soap based extrusion bars to simultaneously facilitate the bar processing, enhance the mildness of the bar formulation, and enhance the creaminess of the bar lather performance.
In brief, the patents mentioned above, alone or in combination, fail to teach or suggest identifying and incorporating a specific type of solid amphoteric surfactants in personal washing bars, which simultaneously accomplishes the following when compared with incorporating other types of amphoteric surfactants:
(1) dramatically improves the mixing-drying and extrusion processes; PA1 (2) significantly improve the bar mildness when compared to other types of amphoterics incorporated in bars; PA1 (3) improve the bar lather without negatively affecting the bar firmness. PA1 (1) bars containing high levels of the amphoteric surfactant can be processed using the current extrusion-stamping technology as described in the Methodology section; PA1 (2) mixing/drying cycle is significantly reduced; PA1 (3) skin irritation is significantly reduced when compared to formulations containing other types of amphoteric surfactants; PA1 (4) bar hardness is not negatively affected, and lather is improved. PA1 (1) bars containing high levels of said amphoteric surfactants can be processed using the current extrusion-stamping technology, which is in contrast to the processing difficulties encountered when comparable levels of other types of amphoteric surfactants are included in the bars; PA1 (2) mixing/drying cycle is significantly reduced; PA1 (3) skin irritation is significantly reduced when compared to formulations containing other types of amphoteric surfactants; PA1 (4) bar hardness is not negatively affected, and lather is improved. PA1 (1) about 15-97% of lathering anionic surfactants; PA1 (2) 0-70% organic and inorganic structurants and fillers; PA1 (3) 0-30% skin emollients and moisturizers; PA1 (4) 0-5% hygroscopic amphoteric surfactants outside the definition of (5); and PA1 (5) 3 to 25% of a specific amphoteric surfactant which is in a solid form at a temperature range between 18.degree. C. and 60.degree. C.
Surprisingly and unexpectedly, applicants have found that all these goals can be simultaneously achieved by including a specific type of solid amphoteric surfactants in extruded bars. That is, by carefully selecting a solid amphoteric surfactant (i.e, with specific melting temperature or glass transition temperature ranges in the solid regime, something which is rare in the amphoteric surfactant class) and by incorporating a significant level of said amphoteric surfactant in personal washing bars, four goals are simultaneously achieved: